Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density, driving voltage, long lifespan and low self-discharge are commercially available and widely used.
In addition, increased interest in environmental issues has brought about a great deal of research associated with electric vehicles (EVs) and hybrid electric vehicles (HEVs) as substitutes for vehicles, such as gasoline vehicles and diesel vehicles, using fossil fuels which are major causes of air pollution. Nickel metal hydride (Ni-MH) secondary batteries are generally used as power sources of electric vehicles (EVs), hybrid electric vehicles (HEVs) and the like. However, a great deal of study associated with use of lithium secondary batteries, high energy density high discharge voltage and power stability is currently underway and some are commercially available.
In particular, lithium secondary batteries used for electric vehicles should have high energy density, exhibit great power within a short time and be used for 10 years or longer under harsh conditions in which charge and discharge at a high current are repeated for a short time, thus requiring considerably superior safety and long lifespan, as compared to conventional small lithium secondary batteries.
Lithium-containing cobalt oxide (LiCoO2) with a layered structure is generally used as a conventional cathode active material for small-sized lithium ion secondary batteries and use of lithium-containing manganese oxides such as LiMnO2 having a layered-crystal structure and LiMn2O4 having a spinel-crystal structure, and lithium-containing nickel oxide (LiNiO2) is also considered.
Of these cathode active materials, LiCoO2 is the most generally used due to superior lifespan characteristics and charge/discharge efficiency, but has disadvantages of low structural stability and limited price competitiveness due to high price caused by resource limitation of cobalt used as a raw material.
Lithium manganese oxides such as LiMnO2 and LiMn2O4 have advantages of superior thermal safety and low cost, but have disadvantages of low capacity and poor high-temperature characteristics.
In addition, LiNiO2-based cathode active materials exhibit high battery discharge capacity, but have disadvantages in that it is considerably difficult to synthesize LiNiO2-based cathode active materials through simple solid phase reaction, essential dopants are required and rate characteristics are poor.